Combining two IMAGES so they fade into one another in a gradient or transition
I'd say your intuition is correct, you don't even need to remake your materials.
Just use a single plane, and use a gradient texture as a mix value in the Factor socket of a Mix Shader node to fade between the two.

Then copy paste your two materials into the same shader tree and mix them together.

You may use a color ramp node to control the transition.